The present invention relates to a demodulator comprising a phase-locked loop. A particularly interesting field of application of this invention is the field of satellite television.
Actually, the spatial links preferably utilize angular modulation (phase or frequency) for two main reasons: the power limitation of the transmitters installed on board satellites and the necessary for the highest possible background noise immunity. On reception of signals coming from a satellite, three main parameters must be optimized to make the best possible use of these signals: the gain of the receiving aerial, the noise factor of the high-frequency input amplifier and the demodulation threshold. The control of this third parameter, the demodulation threshold, is the object of the present invention.
Whatever the use of the demodulator, the curve P.sub.SV /P.sub.BV =f(P.sub.HF /P.sub.BC), where P.sub.SV, P.sub.BV, P.sub.HF and P.sub.BC represent the power of the demodulated video signal, the video noise power, the power of the HF carrier and the HF noise power in the channel, respectively, has a bend beyond which the ratios P.sub.HF /P.sub.BC and P.sub.SV /P.sub.BV evolve substantially proportionally, but below which P.sub.SV /P.sub.BV degrades very rapidly when P.sub.HF /P.sub.BC decreases. The value of P.sub.HF /P.sub.BC corresponding to this bend, called the demodulation threshold, depends on the type of demodulator used.